1. Field of the Invention
The present invention relates generally to post-tensioned concrete structures, and more particularly to a system and method for inspection and documentation of tendon terminations in post-tensioned concrete structures.
2. Description of Related Art
Concrete is a well-known construction material that has been in use in the construction of concrete structures for thousands of years. Post-tensioning is a type of steel reinforcement for concrete structures. Compressive forces are induced in a concrete structure by tensioning steel tendons comprised of strands or bars placed in ducts embedded in the concrete. The tendons are installed after the concrete has been placed and sufficiently cured to a prescribed initial compressive strength. A hydraulic jack is attached to one or both ends of the tendon and pressurized to a predetermined value while bearing against the end of the concrete structure. This induces a predetermined force in the tendon and the tendon elongates elastically under this force. After jacking to the full required force, the tendon is transferred from the jack to an end anchorage.
Tendons made up of strands are secured by steel wedges that grip each strand and seat thinly in a wedge plate disposed within a recess, known as a grout pocket. The wedge plate itself carries all the strands and bears on a steel anchorage. The anchorage may be a simple steel bearing plate or may be a special casting with two or three concentric bearing surfaces that transfer the tendon force to the concrete. The protruding “tails” or strands or bars of permanent tendons are cut off using an abrasive disc saw or plasma cutter after stressing. Tendons are then grouted using a cementitious based grout which fills the grout pocket.
Post tension tendons are protected from corrosion along most of the lengths thereof by a coating of grout, grease, and/or a plastic sheath depending on the application. The bare anchorage and terminal ends of the tendons, however, are typically protected by installation of grease and a grease cap, and the grout pocket is further sealed by concrete grout. If the tendon ends are not adequately protected, significant corrosion problems can arise. Typically, the end of the tendon must be cut off a specified distance from the recessed anchorage assembly to allow for sufficient grout coverage to prevent corrosion-inducing moisture penetration. The tail-ends of the tendons are typically required to extend within the grout pocket a specified distance (e.g. 0.5″, 0.75″ or 1.25″) beyond the anchorage per code and/or industry specification.
In order to verify that the concrete structure has been built to proper specification, construction personnel are often required to verify and document that the tendons have been terminated at the specified length. This process has proven difficult, time consuming, and hazardous, particularly with multistory structures, as the grout pockets are typically formed on the exterior of the structure and inspection will typically occur before walls are erected. As should be apparent, obtaining visual confirmation of tendon terminations recessed within grout pockets formed on the exterior surfaces of partially constructed concrete buildings presents numerous challenges and hazards.
Accordingly, there exists a need for an apparatus and method for inspecting, verifying, and documenting tendon terminations in post-tensioned concrete structures.